Oriented heat-sealable multilayer packaging film

ABSTRACT

A three layered heat shrinkable, heat sealable thermoplastic packaging film having a desired combination of physical characteristics is disclosed. The film may be utilized to package a wide variety of items. A preferred embodiment of the film comprises a core layer consisting essentially of a linear low density polyethylene and two surface layers comprising a three component blend of (1) a linear low density polyethylene, (2) a linear medium density polyethylene and (3) a copolymer of ethylene and vinyl acetate.

FIELD OF THE INVENTION

The present invention relates to a heat sealable and heat shrinkable,thermoplastic film which may be utilized to package a wide variety ofitems. A preferred embodiment of the present invention comprises apalindromic three layer film comprising an interior layer of a linearlow density polyethylene and two surface layers comprising a threecomponent blend of (1) a linear low density polyethylene, (2), a linearmedium density polyethylene and (3) an ethylene vinyl acetate copolymerwhereby a desired combination of physical characteristics beneficiallyresults.

BACKGROUND OF THE INVENTION

The present invention is directed to new and useful multi-layer heatshrinkable film formulations. One distinguishing feature of a heatshrink film is the film's ability, upon exposure to a certaintemperature, to shrink or, if restrained from shrinking, to generateshrink tension within the film.

The manufacture of shrink films, is well known in the art, may begenerally accomplished by extrusion (single layer films) or coextrusion(multi-layer films) of thermoplastic resinous materials which have beenheated to their flow or melting point from an extrusion or coextrusiondie in, for example, either tubular or planer (sheet) form. After a postextrusion quenching to cool by, for example, the well-known cascadingwater method, the relatively thick "tape" extrudate is then reheated toa temperature within its orientation temperature range and stretched toorient or align the crystallites and/or molecules of the material. Theorientation temperature range for a given material or materials willvary with the different resinous polymers and/or blends thereof whichcomprise the material. However, the orientation temperature range for agiven thermoplastic material may generally be stated to be below thecrystalline melting point of the material but above the second ordertransition temperature (sometimes referred to as the glass transitionpoint) thereof. Within this temperature range it is easy to effectivelyorient the material.

The terms "orientation" or "oriented" are used herein to generallydescribe the process step and resultant product characteristics obtainedby stretching and immediately cooling a resinous thermoplastic polymericmaterial which has been heated to a temperature within its orientationtemperature range so as to revise the molecular configuration of thematerial by physical alignment of the crystallites and/or molecules ofthe material to improve certain mechanical properties of the film suchas, for example, shrink tension and orientation release stress. Both ofthese properties may be measured in accordance with ASTM D 2838-81. Whenthe stretching force is applied in one direction uniaxial orientationresults. When the stretching force is applied in two directions biaxialorientation results. The term oriented is also used hereininterchangeably with the term "heat shrinkable" with these termsdesignating a material which has been stretched and set by cooling whilesubstantially retaining its stretched dimensions. An oriented (i.e. heatshrinkable) material will tend to return to its original unstretched(unextended) dimensions when heated to an appropriate elevatedtemperature.

Returning to the basic process for manufacturing the film as discussedabove, it can be seen that the film, once extruded (or coextruded if itis a multi-layer film) and initially cooled to by, for example,cascading water quenching, is then reheated to within its orientationtemperature range and oriented by stretching. The stretching to orientmay be accomplished in many ways such as, for example, by "blown bubble"techniques or "tenter framing". These processes are well known to thosein the art and refer to orientation procedures whereby the material isstretched in the cross or transverse direction (TD) and/or in thelongitudinal or machine direction (MD). After being stretched, the filmis quickly quenched while substantially retaining its stretcheddimensions to rapidly cool the film and thus set or lock-in the oriented(aligned) molecular configuration.

Of course, if a film having little or no orientation is desired, e.g.non-oriented or non-heat shrinkable film, the film may be formed from anon-orientable material or, if formed from an orientable material may be"hot blown". In forming a hot blown film the film is not cooledimmediately after extrusion or coextrusion but rather is first stretchedshortly after extrusion while the film is still at an elevatedtemperature above the orientation temperature range of the material.Thereafter, the film is cooled, by well-known methods. Those of skill inthe art are well familiar with this process and the fact that theresulting film has substantially unoriented characteristics. Othermethods for forming unoriented film are well known. Exemplary, is themethod of cast extrusion or cast coextrusion which, likewise, is wellknown to those in the art.

The degree of stretching controls the degree or amount of orientationpresent in a given film. Greater degrees of orientation are generallyevidenced by, for example, increased values of shrink tension andorientation release stress. That is, generally speaking, for filmsmanufactured from the same material under otherwise similar conditions,those films which have been stretched, e.g. oriented, to a greaterextent will exhibit larger values for free shrink, shrink tension and/ororientation release stress. As stated above, the last two values are tobe measured in accordance with ASTM-D-2838-81. The first value should bemeasured in accordance with ASTM D 2732-70 (reapproved 1976).

After setting the stretch-oriented molecular configuration the film maythen be stored in rolls and utilized to tightly package a wide varietyof items. In this regard, the product to be packaged may first beenclosed in the heat shrinkable material by heat sealing the shrink filmto itself where necessary and appropriate to form a pouch or bag andthen inserting the product therein and closing the bag or pouch by heatsealing or other appropriate means such as, for example, clipping. Ifthe material was manufactured by "blown bubble" techniques the materialmay still be in tubular form or it may have been slit and opened up toform a sheet of film material. Alternatively, a sheet of the materialmay be utilized to over-wrap the product which may be in a tray. Thesepackaging methods are all well known to those of skill in the art.Thereafter, the enclosed product may be subjected to elevatedtemperatures by, for example, passing the enclosed product through a hotair of hot water tunnel. This causes the enclosing film to shrink aroundthe product to produce a tight wrapping that closely conforms to thecontour of the product. As stated above, the film sheet or tube may beformed into bags or pouches and thereafter utilized to package aproduct. In this case, if the film has been formed as a tube it may bepreferable to first slit the tubular film to form a film sheet andthereafter form the sheet into bags or pouches. Such bag or pouchforming methods, likewise, are well known to those of skill in the art.

The above general outline for manufacturing of films is not meant to beall inclusive since such processes are well known to those in the art.For example, see U.S. Pat. Nos. 4,274,900; 4,229,241; 4,194,039;4,188,443; 4,048,428; 3,821,182 and 3,022,543. The disclosures of thesepatents are generally representative of such processes and are herebyincorporated by reference.

Alternative methods of producing films of this type are known to thosein the art. One well-known alternative is the method of forming amulti-layer film by an extrusion coating rather than by an extrusion orcoextrusion process as was discussed above. In extrusion coating a firsttubular layer is extruded and thereafter an additional layer or layersis sequentially coated onto the outer surface of the first tubular layeror a successive layer. Exemplary of this method is U.S. Pat. No.3,741,253. This patent is generally representative of an extrusioncoating process and is hereby incorporated by reference.

Many other process variations for forming films are well known to thosein the art. For example, multiple layers may be first coextruded withadditional layers thereafter being extrusion coated thereon. Or twomulti-layer tubes may be coextruded with one of the tubes thereafterbeing extrusion coated or laminated onto the other. The extrusioncoating method of film formation may be preferable to coextruding theentire film when it is desired to subject one or more layers of the filmto a treatment which may be harmful to one or more of the other layers.Exemplary of such a situation is a case where it is desired to irradiateone or more layers of a film containing an oxygen barrier layercomprised of one or more copolymers of vinylidene chloride and vinylchloride. Those of skill in the art generally recognize that irradiationis generally harmful to such oxygen barrier layer compositions.Accordingly, by means of extrusion coating, one may first extrude orcoextrude a first layer or layers, subject that layers or layers toirradiation and thereafter extrusion coat the oxygen barrier layer and,for that matter, other layers sequentially onto the outer surface of theextruded previously irradiated tube. This sequence allows for theirradiation cross-linking of the first layer or layers withoutsubjecting the oxygen barrier layer or other sequentially added layersto the harmful effects thereof.

Irradiation of an entire film or a layer or layers thereof may bedesired so as to improve the film's resistance to abuse and/or punctureand other physical characteristics. It is generally well known in theart that irradiation of certain film materials results in thecross-linking of the polymeric molecular chains contained therein andthat such action generally results in a material having improved abuseresistance. When irradiation is employed to accomplish thecross-linking, it may be accomplished by the use of high energyirradiation using electrons, X-rays, gamma rays, beta rays, etc.Preferably, electrons are employed of at least about 10⁴ electron voltenergy. The irradiation source can be a Van de Graaff electronaccelerator, e.g. one operated, for example, at about 2,000,000 voltswith a power output of about 500 watts. Alternatively, there can beemployed other sources of high energy electrons such as the GeneralElectric 2,000,000 volt resonant transformer or the corresponding1,000,000 volt, 4 kilowatt, resonant transformer. The voltage can beadjusted to appropriate levels which may be, for example, 1,000,000 or2,000,000 or 3,000,000 or 6,000,000 or higher or lower. Other apparatusfor irradiating films are known to those of skill in the art. Theirradiation is usually carried out at between one megarad and 75megarads, with a preferred range of 8 megarads to 20 megarads.Irradiation can be carried out conveniently at room temperature,although higher and lower temperatures, for example, 0° C. to 60° C. maybe employed.

Cross-linking may also be accomplished chemically through utilization ofperoxides as is well known to those of skill in the art. A generaldiscussion of cross-linking can be found at pages 331 to 414 of volume 4of the Encyclopedia of Polymer Science and Technology, Plastics, Resins,Rubbers, Fibers published by John Wiley & Sons, Inc. and copyrighted in1966. This document has a Library of Congress Catalog Card Number of64-22188.

Another possible processing variation is the application of a fine mistof a silicone or anti-fog spray to the interior of the freshly extrudedtubular material to improve the further processability of the tubularmaterial. A method and apparatus for accomplishing such internalapplication is disclosed in a European patent application underpublication no. of 0071349A2. This document was published on or aboutFeb. 9, 1983.

The polyolefin family of shrink films and, in particular, thepolyethylene family of shrink films provide a wide range of physical andperformance characteristics such as, for example, shrink force (theamount of force that a film exerts per unit area of its cross-sectionduring shrinkage), the degree of free shrink (the reduction in lineardimension in a specified direction that a material undergoes whensubjected to elevated temperatures while unrestrained), tensil strength(the highest force that can be applied to a unit area of film before itbegins to tear apart), heat sealability (the ability of the film to heatseal to itself or another given surface), shrink temperature curve (therelationship of shrink to temperature), tear initiation and tearresistance (the force at which a film will begin to tear and continue totear), optics (gloss, haze and transparency of material), elongation(the degree the film will stretch or elongate at room temperature),elastic memory (the degree a film will return to its originalunstretched (unelongated) dimension after having been elongated at roomtemperature), and dimensional stability (the ability of the film toretain its original dimensions under different types of storageconditions). Film characteristics play an important role in theselection of a particular film and they may differ for each filmapplication.

In view of the many above-discussed physical characteristics which areassociated with polyolefin films and films containing a polyolefinconstituent and in further view of the numerous applications with whichthese films have already been associated and those to which they may beapplied in the future, it is readily discernable that the need for everimproving any or all of the above described physical characteristics orcombinations thereof in these films is great, and, naturally, ongoing.In particular, the quest for highly oriented films which may be utilizedas packaging materials is ever ongoing. One very desirable trait that ashrink film should possess is improved shrinkage (percent free shrink)at lower temperatures. This allows the hot air tunnels or hot waterbaths which are utilized in the industry to shrink a bag or packageformed from the film into close configuration with the packaged articleto be operated at lower temperatures and results in energy savings.Another desirable trait a shrink film should have is a relativelygradual and linear shrink/temperature curve. The shrinkage a given filmwill undergo when heated to a given temperature can be more acuratelyestimated and controlled when the shrink/temperature curve (ratio offree shrink versus degrees Fahrenheit) of a film is approximatelygradual and linear. Yet another trait a shrink film should have is goodabuse resistance. This trait is generally evidenced by improved ballburst and tear propagation values.

Prior art films utilizing linear polyethylene materials and blendsthereof are known to those of skill in the art. Exemplary multilayerprior art films having a core layer of linear low density polyethylenematerial are U.S. Pat. No. 4,364,981 to Horner which discusses a threelayer film having a core layer of low pressure, low density polyethylene(LLDPE) and outer layers of high pressure, low density polyethylene(conventional low density polethylene) and U.S. Pat. No. 4,399,180 toBriggs which discusses a stretch-wrap film having a core layer of linearlow density polyethylene with a layer, on at least one side, comprisinga highly branched low density polyethylene. U.S. Pat. No. 4,399,173 toAnthony discusses a multilayer film comprising a core layer of low meltindex, low pressure, low density polyethylene and two outer layers of ahigh melt index, low pressure, low density polyethylene. U.S. Pat. No.4,425,268 to Cooper discloses a composition adapted for processing intostretch-wrap film. Generally, the Cooper composition comprises a blendof an ethylene vinyl acetate copolymer and a linear low densitypolyethylene material. The material may also contain a tackifier.

An exemplary prior art monolayer film is a monolayer film comprising ablend of (a) about 50%, by weight, of a conventional low densitypolyethylene resin having a density of about 0.922 grams per cubiccentimeter at 23° C. and a melt flow index of about 1.8-2.0 grams perten minutes (ASTM D 1238, condition E) and (b) about 50%, by weight, ofa conventional high density polyethylene resin having a density of about0.953 grams per cubic centimeter and a melt flow index of from about 5.4to about 7.4 grams per ten minutes. The lower density resin may beobtained from the El Paso Polyolefins Company under the tradedesignation of Rexene PE-109-CS-52 resin. The high density material maybe obtained from the U.S. Industrial Chemicals Co. under the tradedesignation Petrothene LY660 resin. The monolayer film may also containappropriate antioxidants and other additives, as necessary. This priorart film will hereafter be referred to as prior art film "A".

Another exemplary prior art monolayer film is a film comprising about100%, by weight, of a linear medium density polyethylene material havinga density of about 0.935 grams per cubic centimeters at 23° C. and amelt flow index of about 2.55 grams per ten minutes (ASTM D 1238,condition E). This material may be obtained from the Dow ChemicalCompany undet the trade designation Dowlex 2037 resin. The film maycontain small amounts of additives and, preferably, comprises a surfacecoating of a polyorganosiloxane material. The surface coating is aprocessing aid applied to the inner surface of the material shortlyafter its extrusion in tubular form. This prior art monolayer film willhereafter be referred to as prior art film "B".

OBJECTS OF THE PRESENT INVENTION

Accordingly, it is a general object of the present invention to providea heat sealable and heat shrinkable packaging film.

It is another object of the present invention to provide a heatshrinkable film having a desired new and improved combination ofphysical characteristics such as, for example, a high degree oforientation or heat shrinkability combined with good puncture and tearresistance along with moderate elongation and elastic memory.

It is an additional object of the present invention to provide a heatshrinkable film having a high degree of orientation and a desirableshrink/temperature curve. In particular the shrink/temperature curveshould have a more constant, gradual and linear slope over a relativelybroad temperature range of, for example, 200° F. to 260° F. Thisprovides for a lower and/or broader shrink tunnel temperature operatingrange.

A further object of the present invention is to provide a heatshrinkable film having a high degree of orientation and also having goodball burst and tear propagation values.

Another object of the present invention is to provide a three layer heatshrink film having a core layer comprising a linear low densitypolyethylene and two surface layers comprising a three component blendof (1) a linear low density polyethylene, (2) a linear medium densitypolyethylene and (3) an ethylene vinyl acetate copolymer.

Yet another object of the present invention is to provide a three layerfilm having a core layer consisting of essentially of a linear lowdensity polyethylene and two surface layers consisting essentially of athree component blend of (1) a linear low density polyethylene, (2) alinear medium density polyethylene and (3) an ethylene vinyl acetatecopolymer.

An even further object of the present invention is to provide a threelayer heat sealable heat shrink film comprising a core layer comprisinga linear low density polyethylene and two surface layers comprising athree component blend of (1) from about 40% to about 60%, by weight, oflinear low density polyethylene, (2) from about 20% to about 30%, byweight, of linear medium density polyethylene and (3) from about 20% toabout 30%, by weight, of an ethylene vinyl acetate copolymer.

One other object of the present invention is to provide a three layerheat sealable heat shrink film comprising a core layer consistingessentially of a linear low density polyethylene and two surface layersconsisting essentially of a three component blend of (1) from about 40%to about 60%, by weight, of linear low density polyethylene, (2) fromabout 20% to about 30%, by weight, of linear medium density polyethyleneand (3) from about 20% to about 30%, by weight, of an ethylene vinylacetate copolymer.

Yet a further object of the present invention is to provide a threelayer heat shrink film adapted for use as a heat sealable packaging filmand which comprises a core layer consisting essentially of a linear lowdensity polyethylene and two surface layers consisting essentially of athree component blend of (1) about 50%, by weight, linear low densitypolyethylene (2) about 25%, by weight, of linear medium densitypolyethylene and (3) about 25%, by weight, of an ethylene vinyl acetatecopolymer.

Still further objects and the broad scope of applicability of thepresent invention will become apparent to those of ordinary skill in theart from the details disclosed hereinafter. However, it should beunderstood that the following detailed description which indicates thepresently preferred embodiment of the present invention is only givenfor purposes of illustration since various changes and modificationswell within the scope of the present invention will become apparent tothose of ordinary skill in the art in view of the following detaileddescription.

DEFINITIONS

Unless specifically set forth and defined or otherwise limited, theterms "polymer" or "polymer resin" as used herein generally include, butare not limited to, homopolymers, copolymers, such as, for exampleblock, graft, random and alternating copolymers, terpolymers etc. andblends and modifications thereof. Furthermore, unless otherwisespecifically limited the terms "polymer" or "polymer resin" shallinclude all possible symmetrical structures of the material. Thesestructures include, but are not limited to, isotactic, syndiotactic andrandom symmetries.

The terms "melt flow" as used herein is the amount, in grams, of athermoplastic resin which can be forced through a given orifice under aspecified pressure and temperature within ten minutes pursuant to ASTM D1238-79. The term "melt flow index" as used herein is the amount, ingrams, of a thermoplastic resin which can be forced through a givenorifice within ten minutes pursuant to condition E of ASTM D 1238-79.

The terms "surface" or "surface layer" or "skin" or "skin layer" as usedherein means a layer of a multi-layer film which comprises a surfacethereof.

The term "interior" or "interior layer" as used herein refers to a layerof a multi-layer film which is not a skin or surface layer of the film.

The term "core" or "core layer" as used herein refers to an interiorlayer of a multi-layer film having an odd number of layers wherein thesame number of layers is present on either side of the core layer.

The term "intermediate" or "intermediate layer" as used herein refers toan interior layer of a multi-layer film which is positioned between acore layer and a surface layer of said film.

The term "palindromic" film as used herein refers to a multilayer filmthe layer configuration of which is substantially symmetrical. Examplesof palindromic films would be film having the following layerconfigurations: (1) A/B/A, (2) A/B/B/A, (3) A/B/C/B/A, etc. An exampleof a non-palindromic film layer configuration would be a film having alayer configuration of A/B/C/A.

The term polyolefin as used herein refers to polymers of relativelysimple olefins such as, for example, ethylene, propylene, butenes,isoprenes and pentenes; including, but not limited to, homopolymers,copolymers, blends and modifications of such relatively simple olefins.

The term "polyethylene" as used herein refers to a family of resinsobtained by polymerizing the gas ethylene, C₂ H₄. By varying thecatalysts and methods of polymerization, properties such as density,melt index, crystallinity, degree of branching and cross-linking,molecular weight and molecular weight distribution can be regulated overwide ranges. Further modifications are obtained by copolymerization,chlorination, and compounding additives. Low molecular weight polymersof ethylene are fluids used as lubricants; medium weight polymers arewaxes miscible with paraffin; and the high molecular weight polymers(generally over 6,000) are resins generally used in the plasticsindustry. Polyethylenes having densities ranging from about 0.900 gramsper cubic centimeter to about 0.925 grams per cubic centimeter arecalled low density polyethylenes with those having densities from about0.926 grams per cubic centimeter to about 0.940 grams per cubiccentimeter being called medium density polyethylenes; and those havingdensities of from about 0.941 grams per cubic centimeter to about 0.965grams per cubic centimeter and over are called high densitypolyethylenes. Conventional low density types of polyethylenes areusually polymerized at high pressures and temperatures whereas the highdensity types are usually polymerized at relatively low temperatures andpressures. The molecular structure of conventional low densitypolyethylenes is highly branched. While conventional medium densitypolyethylenes possess a molecular structure which is branched, thedegree of branching is less than that of conventional low densitypolyethylenes. The molecular structure of high density polyethylenespossess little or no side branching.

The terms "linear low" or "linear medium density polyethylene" as usedherein refer to copolymers of ethylene with one or more comonomersselected from C₄ to C₁₀ alpha olefins such as butene-1, octene, etc. inwhich the molecules thereof comprise long chains with few side chainsbranches or cross-linked structures. This molecular structure is to becontrasted with conventional low or medium density polyethylenes whichare more highly branched than their respective linear couterparts.Moreover, the side branching which is present in linear low or linearmedium density polyethylenes will be short as compared to the respectivenon-linear polyethylenes. The molecular chains of a linear polymer maybe intertwined, but the forces tending to hold the molecules togetherare believed to be physical rather than chemical and thus may beweakened by energy applied in the form of heat. Linear low densitypolyethylene as defined herein has a density usually in the range offrom about 0.900 grams or less per cubic centimeter to about 0.925 gramsper cubic centimeter and, preferably, the density should be maintainedbetween 0.916 grams per cubic centimeter to 0.925 grams per cubiccentimeter. Linear medium density polyethylene, as defined herein, has adensity usually in the range of from about 0.926 grams per cubiccentimeter to about 0.941 grams per cubic centimeter. The melt flowindex of linear low and medium density polyethylenes generally rangesfrom between about 0.1 to about 10 grams per ten minutes and preferablybetween from about 0.5 to about 3.0 grams per ten minutes. Linear lowand linear medium density polyethylene resins of this type arecommercially available and are manufactured in low pressure vapor phaseand liquid phase processes using transition metal catalysts.

The term "ethylene vinyl acetate copolymer" (EVA) as used herein refersto a copolymer formed from ethylene and vinyl acetate monomers whereinthe ethylene derived units in the copolymer are present in major amountsand the vinyl acetate derived units in the copolymer are present inminor amounts.

An "oriented" or "heat shrinkable" material is defined herein as amaterial which, when heated to an appropriate temperature above roomtemperature (for example 96° C.), will have a free shrink of 5% orgreater in at least one linear direction.

All compositional percentages used herein are calculated on a "byweight" basis.

Density should be measured at 23° Centigrade and in accordance with ASTMD 1505-68 (reapproved 1979).

Free shrink should be measured in accordance with ASTM D 2732.

Shrink tension and orientation release stress should be measured inaccordance with ASTM D 2838-81.

The tensile properties of the film should be measured in accordance withASTM D 882-81.

The elongation properties of the film should be measured in accordancewith ASTM D 638.

The haze and luminous transmittance of the film should be measured inaccordance with ASTM D 1003-61 (reapproved 1971).

The specular gloss of the film should be measured in accordance withASTM D 2457-70 (reapproved 1977).

The tear propagation of the film should be measured in acordance withASTM D 1938-67 (reapproved 1978).

The impact resistance of the film should be measured in accordance withASTM D 3420-80.

One method for determining whether a material is "cross-linked" is toreflux the material in boiling toluene or xylene, as appropriate, forforty (40) hours. If a weight percent residue of at least 5 percentremains the material is deemed to be cross-linked. A procedure fordetermining whether a material is cross-linked vel non is to reflux 0.4gram of the material in boiling toluene or another appropriate solvent,for example xylene, for twenty (20) hours. If no insoluble residue (gel)remains the material may not be cross-linked. However, this should beconfirmed by the "melt flow" procedure, below. If, after twenty (20)hours of refluxing insoluble residue (gel) remains the material isrefluxed under the same conditions for another twenty (20) hours. Ifmore than 5 weight percent of the material remains upon conclusion ofthe second refluxing the material is considered to be cross-linked.Preferably, at least two replicates are utilized. Another method wherebycross-linking vel non and the degree of cross-linking can be determinedis by ASTM-D-2765-68 (Reapproved 1978). Yet another method fordetermining whether a material is cross-linked vel non is to determinethe melt flow of the material in accordance with ASTM D 1238-79 at 230°Centigrade and while utilizing a 21,600 gram load. Materials having amelt flow of greater than 75 grams per ten minutes are deemednon-cross-linked. This method should be utilized to confirm the "gel"method described above whenever the remaining insoluble residue (gelcntent) is less than 5% by weight, since some cross-linked materialswill evidence a residual gel content of less than 5 weight percent. Ifthe cross-linking is accomplished by irradiation of the film the amountof ionizing radiation which has been absorbed by a known film materialcan be calculated by comparing the weight percent of insoluble material(gel) remaining after refluxing the sample to the weight percents of gelremaining after refluxing standards of the same material which have beenirradiated to different known degrees. Those of skill in the art alsorecognize that a correlation exists between the amount of ionizingirradiation absorbed and the melt flow of a material. Accordingly, theamount of ionizing irradiation which a material has absorbed may bedetermined by comparing the melt flow of the material to the melt flowof samples of the same material which have been irradiated to differentknown degrees.

The term "crystalline" or "crystalline polymer" material, etc. as usedherein refers to a polymeric material which is composed of molecularchains which are so constructed that they can pack together well inordered arrangements. The finite volume throughout which the orderextends is designated by the term "crystallite" with the surroundingdisordered regions, if any, being designated by the term "amorphous".The crystallites are denser than the surrounding amorphous regions ofthe material and also have a higher refractive index. If a crystallinematerial is oriented the crystallites become generally aligned with eachother. Three well known methods for determining the degree ofcrystallinity are by (1) (a) measuring the specific volume of thespecimen (V), (b) measuring the specific volume of the crystallites (Vc)within the specimen and (c) measuring the specific volume of theamorphous region (Va) contained within the specimen and then utilizingthe equation [% crystallinity=(Va-V)/(Va-Vc)], (2) X-ray diffractionmethods and (3) infrared absorption methods. All of these methods arewell known to those in the art. A general discussion of crystallinitycan be found at pages 449 to 527 of volume 4 of the Encyclopedia ofPolymer Science and Technology, Plastics, Resins, Rubbers, Fiberspublished by John Wiley & Sons, Inc. and copyrighted in 1966. Thisdocument has a Library of Congress Catalogue Card Number of 64-22188.

The term "gauge" is a unit of measure applied to the thickness of filmsor the layers thereof. 100 gauge is equal to 1 l mil which is onethousandth of an inch.

A rad is the quantity of ionizing radiation that results in theabsorption of 100 ergs of energy per gram of a radiated material,regardless of the source of the radiation. A megarad is 10⁶ rads. (MR isan abbreviation for megarad).

SUMMARY OF THE INVENTION

It has been discovered that a flexible, heat shrinkable, heat sealablethermoplastic packaging film having a desirable combination of physicalcharacteristics such as, a high degree of orientation or heatshrinkability, good elongation, good puncture and tear resistance,moderate elastic memory e.g. snap-back or elastic recovery and adesirable shrink/temperature curve has been achieved by the flexiblefilm of the present invention.

This film comprises an interior layer comprising a linear low densitypolyethylene and two surface layers comprising a three component blendof (1) a linear low density polyethylene, (2) a linear medium densitypolyethylene and (3) an ethylene vinyl acetate copolymer. A preferredembodiment of the film comprises a core layer consisting essentially ofa linear low density polyethylene and two surface layers consistingessentially of a three component blend of (1) from about 40% to about60%, by weight, of a linear low density polyethylene, (2) from 20% toabout 30%, by weight, linear medium density polyethylene and (3) fromabout 20% to about 30%, by weight, of an ethylene vinyl acetatecopolymer. The most preferred embodiment of the present invention is athree layered film comprising a core layer which consists essentially ofa linear low density polyethylene which is a copolymer of ethylene andoctene having a density of about 0.920 grams per cubic centimeter. Thetwo surface layers of the most preferred embodiment consist essentiallyof a three component blend of (1) about 50%, by weight, of a linear lowdensity polyethylene which is a copolymer of ethylene and octene havinga density of about 0.920 grams per cubic centimeter, (2) about 25%, byweight, of a linear medium density polyethylene having a density ofabout 0.935 gram per cubic centimeter and (3) about 25%, by weight, ofan ethylene vinyl acetate copolymer having about 3.6% vinyl acetatederived units and a density of from about 0.9232-0.9250 grams per cubiccentimeter.

The film is both stretched, e.g. biaxially oriented, and cross-linked.Preferably the film is cross-linked by irradiation with from about 1.0to about 6.0 MR. A more preferable degree of cross-linking isaccomplished by irradiation in the range of from about 1.5 to about 3.5MR. The most preferable degree of cross-linking is accomplished byirradiation with about 2.5 MR. As a result of the cross-linking the filmwill possess a melt flow of from about 1.0 to about 10 grams per tenminutes when measured at 230° C. under a total load of 21,600 grams inaccordance ASTM D 1238-79.

The degree of stretching to achieve the appropriate degree of biaxialorientation and associated physical characteristics is preferably in therange of from about 3.0 to about 6.0 or greater times the originaldimensions in both the transverse (TD) and longitudinal (MD) directions.More preferably the film is a highly oriented film possessing a degreeof stretching which is greater than about 4.5 times the originaldimensions in both the transverse and longitudinal directions. Even morepreferably the degree of stretching is from about 4.5 times to about 5.5the original dimensions in both the transverse and longitudinaldirections. The most preferred degree of stretching, i.e. orientation,is approximately 5.0 times the original dimension in both the transverseand longitudinal directions.

The preferred thickness ratio of a skin layer to the core layer rangesfrom about 1 to 1 to about 1 to 4. Preferably the thicknesses of the twoskin layers are substantially equal to each other and the thickness ofeach skin layer is about one-half the thickness of the core layer. Totalfilm thickness may range from about 0.40 mil to about 2.0 mils. That isfrom about 40 to about 200 gauge. Preferably, total film thickness mayrange from about 50 to about 150 gauge. More preferably, total filmthickness may range from about 60 to 100 gauge.

The multi-layer film may be combined with other polymeric materials forspecific applications. For instance, additional layers may be added oneither or both sides of the film to improve various physicalcharacteristics.

BRIEF DESCRIPTION OF THE DRAWING

FIG. I is a cross-sectional view of a preferred three layered embodimentof the present invention.

FIG. II is a plot of the percent of free shrink in the longitudinaldirection versus temperature, in degrees F. for embodiments I, II andIII and prior films A and B.

FIG. III is a plot of the percent of free shrink in the transversedirection versus temperature, in degrees F. for embodiments I, II andIII and prior films A and B, below.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. I, which is a cross-sectional view of a preferredthree layered embodiment of the present invention, it is seen that thisembodiment comprises a core layer 1 and two skin or surface layers 2 and3. The preferred thickness ratio of the three layers of 1/2/1 isdemonstrated in FIG. I. Preferred core layer 1 formulations comprise oneor more linear low density polyethylene materials. Preferably, corelayer 1 consists essentially of a linear low density polyethylenematerial which is a copolymer of ethylene and octene having a density offrom about 0.918 to about 0.922 grams per cubic centimeter. Mostpreferably, core layer 1 consists essentially of a linear low densitypolyethylene material which is a copolymer of ethylene and octene havinga density of about 0.920 grams per cubic centimeter.

Experimentation has revealed an especially preferred core layer materialwhich may be obtained from the Dow Chemical Company under the tradedesignation Dowlex 2045. This material has a density of 23° C. of about0.920 grams per cubic centimeter and a melt flow rate (measured byASTM-D-1238, E-28) of about 0.7-1.2 grams per ten minutes. Other linearlow density polyethylene materials or blends thereof may be utilized toform core layer 1.

Returning to FIG. I and, in particular, to surface layers 2 and 3experimentation has determined that a preferred surface layerformulation should comprise a three component blend of (1) a linear lowdensity polyethylene material, (2) a linear medium density polyethylenematerial and (3) an ethylene vinyl acetate copolymer.

Preferably the formulation of the two skin layers comprises a threecomponent blend of (1) from about 40% to about 60%, by weight, of alinear low density polethylene material, (2) from about 20% to about30%, by weight, of a linear medium density polyethylene material and (3)from about 20% to about 30%, by weight, of an ethylene vinyl acetatecopolymer. Even more preferably the surface layers of the film shouldcomprise a three component blend of (1) from about 45% to about 55%, byweight, of a linear low density polyethylene material, (2) from about23% to about 27%, by weight, of a linear medium density polyethylenematerial and (3) from about 23% to about 27%, by weight, of an ethylenevinyl acetate copolymer. The most preferred skin or surface layerformulation of the present invention consists essentially of a threecomponent blend of (1) about 50%, by weight, of a linear low densitypolyethylene material, (2) about 25%, by weight, of a linear mediumdensity polyethylene material and (3) about 25%, by weight, of anethylene vinyl acetate copolymer.

The same group of linear low density polyethylene materials that may beutilized as core layer materials may also be utilized as the linear lowdensity polyethylene constituent of the surface layers. However, thelinear low density polyethylene material used in the skin layers doesnot have to be the material used in the core layer. A preferred linearlow density polyethylene for utilization in the skin layers is Dowlex2045, discussed above. Preferably the linear medium density polyethyleneof the skin layers has a density of 23° C. of about 0.933 to about 0.937grams per cubic centimeter. More preferably the linear medium densitypolyethylene material has a density at 23° C. of about 0.935 grams percubic centimeter. A preferred linear medium density polyethylenematerial for utilization in the surface layer formulation may beobtained from the Dow Chemical Company under the trade designationDowlex 2037. This material is a copolymer of ethylene and octene and hasa density of 23° C. of about 0.935 grams per cubic centimeter and a flowrate (measured by ASTM-D-1238, condition of E-28) of 2.55±0.35 grams perten minutes. Other linear low density polyethylene and linear mediumdensity polyethylene materials may be utilized as those of skill in theart would readily recognize.

Ethylene vinyl acetate copolymers comprising from about 2%, by weight,to about 18%, by weight, vinyl acetate derived units may be utilized.Preferably the ethylene vinyl acetate copolymer will comprise from about2%, by weight, to about 10%, by weight, of vinyl acetate derived units.Even more preferably the ethylene vinyl acetate copolymer will comprisefrom about 2%, by weight, to about 5%, by weight, of vinyl acetatederived units. The most preferred ethylene vinyl acetate copolymer forutilization in the surface layer formulation may be obtained from the ElPaso Polyolefins Company. This material has a density at 23° C. of from0.9232 to about 0.9250 grams per cubic centimeter and a melt flow(measure by ASTM D 1238, condition E-28) of about 2.0±0.5 grams per tenminutes. The material contains from about 3.3% to about 4.1% vinylacetate derived units. The nominal percent of vinyl acetate derivedunits present in the material is about 3.6%. Other ethylene vinylacetate copolymers may be utilized as those of skill in the art wouldreadily recognize.

Preferably the composition and other parameters of skin layers 2 and 3are substantially the same. However, different linear low densitypolyethylene, linear medium density polyethylene and ethylene vinylacetate copolymers or blends thereof may be utilized for each skinlayer.

Those skilled in the art will readily recognize that all of the byweight percentages disclosed herein are subject to slight variation.Additionally, these percentages may vary slightly as a result of theinclusion or application of additives to the surface layers such as thesilicone mist discussed above or inclusion therein of agents such asslip, antioxidant and anti-block agents. A preferred anti-block agent isa diatomaceous silica, SiO₂, which is available from McCullough &Benton, Inc. under the tradename Superfine Superfloss. This material hasa wet density of about 29.0 pounds per cubic foot a specific gravity ofabout 2.30 and a pH of about 9.5. Other well known antiblock agents maybe utilized. A preferred slip agent is Erucamide (available from HumkoChemical under the tradename Kemamide E). This material is believed tohave an average molecular weight of about 335 and a melting point rangeof from about 72° C. to about 86° C. Other slip agents such asStearamide (available from the Humko Chemical Company under thetradename Kemamide S) and N, N-' Dioleoylethylenediamine (available fromGlyco Chemical under the tradename Acrawax C) may be utilized. Apreferred silicone spray for application to the inner surface of theextruded tube is a liquid polyorganosiloxane manufactured by GeneralElectric under the trade designation General Electric SF18polydimethylsiloxane. A preferred antioxidant and thermal stabilizingagent is tetrakis [methylene 3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane. This material isbelieved to be a symmetrical molecule which includes four stericallyhindered phenolic hydroxyl groups and has a molecular weight of about1178. This material is available from by Ciba-Geigy under the tradedesignation Irganox® 1010.

The general ranges for inclusion of these agents into the surface layers2 and 3 and, in the case of the silicone spray, the application of thespray mist onto the interior surface layer of a tubular extrudate are asfollows:

(1) Diatomaceious Silica:

1000-2000 ppm, preferably

1250-1750 ppm, more preferably

about 1500 ppm, most preferably

(2) Erucamide:

2000-4000 ppm, preferably

2500-3500 more preferably

about 3000 ppm most preferably

(3) Polydimethylisiloxane:

0.5 mg.ft² -and up

(4) Antioxidant:

100-500 ppm, preferably

200-400 ppm, more preferably

about 300 ppm, most preferably

When utilized within the specification and claims of the presentapplication the term "consisting essentially of" is not meant to excludeslight percentage variations or additives and agents of this sort.

Additional layers and/or minor amounts of various additives of the typesdescribed above may be added to the film structure of the presentinvention as desired but care must be taken not to adversely affect thedesired physical propertities and other characteristics of the inventivefilm. It should also be recognized that many resins obtained from theirmanufacturer already contain small amounts of additives of differenttypes.

In the preferred process for making the multi-layer film of the presentinvention the basic steps are coextruding the layers to form amultilayer film, irradiating the film, and then stretching the film tobiaxially orient. These steps and additional desirable steps will beexplained in detail in the paragraphs which follow.

The process begins by blending, as necessary, the raw materials (i.e.polymeric resins) in the proportions and ranges desired as discussedabove. The resins are usually purchased from a supplier in pellet formand can be blended in any one of a number of commercially availableblenders as is well known in the art. During the blending process anyadditives and/or agents which are desired to be utilized are alsoincorporated. The additives may be incorporated into the blend byutilizing a masterbatch containing small percentages of the additives.

The resins and applicable additives and/or agents are then fed to thehoppers of extruders which feed a coextrusion die. For the preferredpalindromic three layer film, having two substantially identical surfacelayers, at least two extruders need to be employed. One for the twosubstantially identical skin or surface layers and one for the corelayer. Additional extruders may be employed if a film havingnon-identical surface layers is desired. The materials are coextruded asa relatively thick tube or "tape" which has an initial diameter andthickness dependent upon the diameter and die gap of the coextrusiondie. The final diameter and thickness of the tubular film is dependentupon the racking ratio, e.g. the stretching ratio. Circular coextrusiondies are well known to those in the art and can be purchased from anumber of manufacturers. As an alternative to tubular coextrusion, slotdies could be used to coextrude the material in sheet form. Well knownsingle or multi-layer extrusion coating processes could also beutilized, if desired.

An additional process step which should be utilized to manufacture thepreferred embodiment of the presently inventive film is to irradiate thetape or unexpanded tubing or sheet by bombarding it with high-energyelectrons from an accelerator to cross-link the materials of the tube.Cross-linking greatly increases the structural strength of the filmand/or the force at which the material can be stretched before tearingapart when the film materials are predominately ethylene such aspolyethylene or ethylene vinyl acetate. Irradiation may also improve theoptical properties of the film and change the properties of the film athigher temperatures. A preferred irradiation dosage level is in therange of from about 1.0 MR to about 6.0 MR. An even more preferred rangeis from about 1.5 MR to about 3.5 MR. The most preferred dosage level isapproximately 2.5 MR. As a result of the cross-linking the final filmwill possess a melt flow of from about 1.0 to about 10.0 grams per tenminutes when measured in accordance with ASTM D 1238-79 at a temperatureof 230° C. under a total load of 21,600 grams. More preferably the flowrate of the film will be from about 2.0 to about 5.0 grams per tenminutes when measured in accordance with ASTM D 1238-79 at a temperatureof 230° C. and a total load of 21,600 grams. Even more preferably themelt flow of the film will be from about 2 to about 4 grams per tenminutes when measured in accordance with ASTM D 1238-79 at 230° C. and atotal load of 21,600 grams. Most preferably the melt flow of the filmwill be about 3.0 grams per ten minutes when measured in accordance withASTM D 1238 at 230° C. and a total load of 21,600 grams.

Following coextrusion, quenching to cool and solidify, and irradiationof the tape, the extruded tape is reheated to its orientationtemperature range and inflated, by application of internal air pressure,into a bubble thereby transforming the narrow tape with thick walls intoa wide film with thin walls of the desired film thickness and width.This process is sometimes referred to as the "trapped bubble technique"of orientation or as "racking". The degree of inflation and subsequentstretching is often referred to as the "racking ratio" or "stretchingratio". For example, a transverse racking or stretching ratio of 2.0would mean that the film had been stretched 2.0 times its originalextruded size in the transverse direction during transverse racking.After stretching, the tubular film is then collapsed into a superimposedlay-flat configuration and wound into rolls often referred to as "millrolls". The racking process orients the film by stretching ittransversely and, to some extent, longitudinally and thus imparts shrinkcapabilities to the film. Additional longitudinal or machine directionracking or stretching may be accomplished by revolving the deflaterollers which aid in the collapsing of the "blown bubble" at a greaterspeed than that of the rollers which serve to transport the reheated"tape" to the racking or blown bubble area. Preferred transverse andlongitudinal stretching ratios of the present film range from about 3.0transverse by about 3.0 longitudinal to about 6.0 or greater transverseby about 6.0 or greater longitudinal. More preferably the degree ofstretching is greater than about 4.5 times the original dimension inboth the transverse and longitudinal directions. Even more preferablythe degree of stretching is from about 4.5 to about 5.5 times theoriginal dimension in both the transverse and longitudinal directions. Aparticularly preferred stretching ratio is about 5.0 in both thetransverse and longitudinal directions.

To further disclose and clarify the scope of the present invention tothose skilled in the art three embodiments of the present invention wereformed by coextrusion, irradiated and stretched (oriented) byapplication of internal air (bubble technique) in accordance with theteachings described above. The embodiments were each three layered filmsirradiated with an average MR of about 2.0±0.5 MR. Each film had anapproximate layer thickness ratio of about 1/2/1. Embodiment I compriseda three layer structure of about "50%, by weight, A+20%, by weight,B+30%, by weight, C//100%, by weight, A//50%, by weight, A+20%, byweight, B+30%, by weight, C". Embodiment II comprised a three layerstructure of about "50%, by weight, A+25%, by weight, B+25%, by weight,C//100%, by weight, A//50%, by weight, A+25%, by weight, B+25%, byweight, C". Embodiment III comprised a three layer structure of about"50%, by weight, A+20%, by weight, B+30%, by weight, C//100%, by weight,D//50%, by weight, A+20%, by weight, B+30%, by weight, C". "A"represents a linear low density polyethylene having a density of about0.920 grams per cubic centimeter (Dowlex 2045). "B" represents a linearmedium density polyethylene having a density of about 0.935 grams percubic centimeter (Dowlex 2037). "C" represents an ethylene vinyl acetatecopolymer having from about 3.3% to about 4.1% vinyl acetate derivedunits and a density of from about 0.9232-0.9250 grams per cubiccentimeter (El Paso PE 204CS95). "D" represents a linear low densitypolyethylene having a density of about 0.920 grams per cubic centimeterat 23° C. and a nominal melt flow index of about 1.0 grams per tenminutes. This linear low density material is believed to be a copolymerof ethylene and butene and may be obtained from the Mobil OilCorporation under the trade designation Mobil MJA- 042. All three filmswere stretched about 4.8 times original in the transverse direction andabout 5.2 times original in the machine or longitudinal direction. Allof the surface layers of all three films also comprised about 3,000parts per millions Erucamide and about 1,500 parts per milliondiatomaceious silica.

Test results for these films are listed below in Table I.

                  TABLE I                                                         ______________________________________                                                  I        II         III                                             ______________________________________                                        Average Gauge.sup.0                                                                       60         60         60                                          Tensile At Break And 73° F. (PSI).sup.1                                Av..sup.2 Long.                                                                           190.4 × 100                                                                        165.2 × 100                                                                        150.3 × 100                           Std. Dev.   10.2 × 100                                                                         10.7 × 100                                                                         5.1 × 100                             95% C.L..sup.3                                                                            16.2 × 100                                                                         17.0 × 100                                                                         8.2 × 100                             Av. Trans.  166.9 × 100                                                                        197.5 × 100                                                                        155.1 × 100                           Std. Dev.   8.1 × 100                                                                          11.2 × 100                                                                         11.6 × 100                            95% C.L.    12.9 × 100                                                                         17.8 × 100                                                                         18.5 × 100                            Elongation At Break And 73° F. (%).sup.4                               Av. Long.   108        113        101                                         Std. Dev.   5          6          6                                           95% C.L.    8          10         10                                          Av. Trans.  127        111        84                                          Std. Dev.   7          6          16                                          95% C.L.    11         9          25                                          Modulus At 73° F. (PSI).sup.5                                          Av. Long.   42.5 × 1000                                                                        49.9 × 1000                                                                        41.8 × 1000                           Std. Dev.   3.2 × 1000                                                                         4.0 × 1000                                                                         2.3 × 1000                            95% C.L.    5.0 × 1000                                                                         6.4 × 1000                                                                         3.6 × 1000                            Av. Trans.  49.6 × 1000                                                                        49.5 × 1000                                                                        51.5 × 1000                           Std. Dev.   5.8 × 1000                                                                         0.8 × 1000                                                                         3.8 × 1000                            95% C.L.    9.3 × 1000                                                                         1.3 × 1000                                                                         6.1 × 1000                            Tear Propagation At 73° F. (grams).sup.6                               Av. Long.   4.71       5.10       6.13                                        Std. Dev.   0.26       0.38       0.15                                        95% C.L.    0.41       0.61       0.24                                        Av. Trans   5.44       7.65       3.55                                        Std. Dev.   1.41       1.03       0.77                                        95% C.L.    2.24       1.65       1.23                                        Ball Burst Impact At 73° F.                                            1.00 In. Diam. Sphere Hd. (cm × kg).sup.7                               Average     21.4       18.5       17.8                                        Std. Dev.   1.7        1.5        4.3                                         95% C.L.    2.7        2.3        6.8                                         Optical Properties At 73° F.                                           Haze (%).sup.8                                                                Avg.        1.7        1.9        1.3                                         Std. Dev.   0.1        0.3        0.1                                         95% C.L.    0.2        0.6        0.2                                         Gloss (45°).sup.9                                                      Avg.        93         91         95                                          Std. Dev.   1          2          1                                           95% C.L.    2          3          2                                           Shrink Properties At 200° F.                                           Free Shrink (%).sup.11                                                        Av. Long.   15         13         14                                          Std. Dev.   1          2          1                                           95% C.L.    1          3          1                                           Av. Trans.  19         18         18                                          Std. Dev.   1          1          0                                           95% C.L.    1          1          0                                           Shrink Force (lbs.).sup.12                                                    Av. Long.   0.248      0.221      0.126                                       Std. Dev.   0.009      0.024      0.009                                       95% C.L.    0.014      0.038      0.015                                       Av. Trans.  0.328      0.324      0.316                                       Std. Dev.   0.006      0.011      0.011                                       95% C.L.    0.010      0.018      0.018                                       Shrink Tension (PSI).sup.13                                                   Av. Long.   339        291        229                                         Std. Dev.   12         11         11                                          95% C.L.    18         17         18                                          Av. Trans.  497        455        538                                         Std. Dev.   6          9          11                                          95% C.L.    9          15         18                                          Shrink Properties At 220° F.                                           Free Shrink (%).sup.11                                                        Av. Long.   25         21         26                                          Std. Dev.   2          2          1                                           95% C.L.    3          4          1                                           Av. Trans.  31         29         30                                          Std. Dev.   1          2          1                                           95% C.L.    1          3          1                                           Shrink Force (lbs.).sup.12                                                    Av. Long.   0.288      0.253      0.190                                       Std. Dev.   0.012      0.020      0.004                                       95% C.L.    0.019      0.032      0.006                                       Av. Trans.  0.353      0.369      0.374                                       Std. Dev.   0.010      0.005      0.005                                       95% C.L.    0.015      0.008      0.008                                       Shrink Tension (PSI).sup.13                                                   Av. Long    406        350        319                                         Std. Dev.   10         11         11                                          95% C.L.    16         18         18                                          Av. Trans.  542        504        608                                         Std. Dev.   6          6          13                                          95% C.L.    10         9          21                                          Shrink Properties At 240° F.                                           Free Shrink (%).sup.11                                                        Av. Long.   60         54         50                                          Std. Dev.   2          2          1                                           95% C.L.    4          3          1                                           Av. Trans.  62         57         58                                          Std. Dev.   1          1          1                                           95% C.L.    2          1          1                                           Shrink Force (lbs.).sup.12                                                    Av. Long.   0.361      0.245      0.309                                       Std. Dev.   0.006      0.008      0.049                                       95% C.L.    0.010      0.013      0.077                                       Av. Trans.  0.355      0.360      0.369                                       Std. Dev.   0.009      0.008      0.013                                       95% C.L.    0.015      0.013      0.021                                       Shrink Tension (PSI).sup.13                                                   Av. Long.   482        392        431                                         Std. Dev.   11         6          33                                          95% C.L.    17         10         53                                          Av. Trans.  473        567        522                                         Std. Dev.   17         15         14                                          95% C.L.    28         23         23                                          Shrink Properties At 260° F.                                           Free Shrink (%).sup.11                                                        Av. Long.   79         78         78                                          Std. Dev.   1          1          1                                           95% C.L.    1          1          1                                           Av. Trans.  78         77         82                                          Std. Dev.   1          1          1                                           95% C.L.    1          2          1                                           Shrink Force (lbs.).sup.12                                                    Av. Long.   0.356      0.264      0.240                                       Std. Dev.   0.018      0.035      0.014                                       95% C.L.    0.028      0.056      0.023                                       Av. Trans.  0.339      0.310      0.296                                       Std. Dev.   0.018      0.014      0.005                                       95% C.L.    0.028      0.022      0.008                                       Shrink Tension (PSI).sup.13                                                   Av. Long.   470        391        374                                         Std. Dev.   15         15         17                                          95% C.L.    25         25         26                                          Av. Trans.  442        483        439                                         Std. Dev.   36         21         13                                          95% C.L.    57         33         20                                          Shrink Properties At 280° F.                                           Free Shrink (%).sup.11                                                        Av. Long.   80         80         80                                          Std. Dev.   1          1          0                                           95% C.L.    1          1          0                                           Av. Trans.  79         76         84                                          Std. Dev.   0          0          1                                           95% C.L.    0          0          1                                           Shrink Force (lbs.).sup.12                                                    Av. Long.   0.273      0.274      0.163                                       Std. Dev.   0.019      0.011      0.018                                       95% C.L.    0.030      0.018      0.029                                       Av. Trans.  0.264      0.253      0.268                                       Std. Dev    0.018      0.009      0.009                                       95% C.L.    0.029      0.014      0.014                                       Shrink Tension (PSI).sup.13                                                   Av. Long.   383        356        299                                         Std. Dev.   12         12         18                                          95% C.L.    20         19         28                                          Av. Trans.  364        360        445                                         Std. Dev.   45         14         14                                          95% C.L.    72         23         22                                          Density at 23 Deg.                                                                        0.9244     0.9250     0.9229                                      C..sup.14 (grams/cubic                                                                    0.9246     0.9258     0.9233                                      centimeter)                                                                   Flow Rate At 230                                                                          2.30       3.13       0.79                                        Deg. C. and 21,600                                                                        2.36       3.63       0.61                                        grams load.sup.15                                                             (grams/ten minutes)                                                                       2.09       3.58       0.56                                        Oxygen Transmis-                                                                          8383.5     8125.0     9602.5                                      sion At 73° F..sup.16                                                              10081.8    11046.9    12833.6                                     [CCSTP/(24 Hrs.,                                                                          9403.5     10174.9    11057.5                                     Sq. M., ATM)]                                                                 Coefficient of Friction                                                       At 73° F. (ASTM SLED).sup.17                                           Out/Out                                                                       Static                                                                        Average     1.341      1.010      0.349                                       Std. Dev.   0.809      0.524      0.097                                       95% C.L.    1.287      0.833      0.154                                       Out/Out                                                                       Kinetic                                                                       Average     0.366      0.444      0.244                                       Std. Dev.   0.120      0.234      0.016                                       95% C.L.    0.190      0.372      0.025                                       In/In                                                                         Static                                                                        Average     Blocked    0.670      0.754                                       Std. Dev.   Blocked    0.376      0.529                                       95% C.L.    Blocked    0.598      0.841                                       Kinetic                                                                       Average     Blocked    0.325      0.290                                       Std. Dev.   Blocked    0.119      0.075                                       95% C.L.    Blocked    0.189      0.120                                       ______________________________________                                         The following footnotes apply to Table                                        .sup.0 100 gauge is equal to 1 mil.                                           .sup.1 ASTM D88281                                                            .sup.2 Where confidence limits (C.L.) are present all values in Table I       are obtained from four (4) replicate measurements.                            .sup.3 C.L. Is Confidence Limit  for example, if the reported average         value was 10 and the 95% C.L. was 2, then if 100 replicate readings were      made, 95 of them would have a value between 8 and 12, inclusive.              .sup.4 ASTM D88281                                                            .sup.5 ASTM D88281                                                            .sup.6 ASTM D193879                                                           .sup.7 ASTM D342080                                                           .sup.8 ASTM D100361 (reapproved 1977)                                         .sup.9 ASTM D245770 (reapproved 1977)                                         .sup.9a Outside surface measurement.                                          .sup.10 ASTM D88281                                                           .sup.11 ASTM D273270 (reapproved 1976)                                        .sup.12 ASTM D283881 (shrink force = shrink tension × film thicknes     in mils × 1000)                                                         .sup.13 ASTM D283881                                                          .sup.14 ASTM D 150568 (reapproved 1979).                                      .sup.15 ASTM D 132879.                                                        .sup.16 ASTM D 398581.                                                        .sup.17 ASTM D 189478.                                                   

The data above demonstrates that the present film has good values fortear propagation and ball burst. Preferred ball burst ranges are fromabout 17 to about 22 centimeters times kilograms and greater. A morepreferable range is greater than from about 19 centimeters timeskilograms. Note also the % free shrink vs. temperature data discussedbelow.

Shrink/temperature curve data was gathered for the above-identifiedembodiments I, II and III. This data was compared to shrink/temperaturecurve values for prior art monolayer films "A" and "B", discussed above.This data is presented in Table II, below and also graphically, in FIGS.II and III.

                  TABLE II                                                        ______________________________________                                        Shrink Properties                                                             Percent Free Shrink Vs. Temperature                                           Film   200° F.                                                                          220° F.                                                                        240° F.                                                                        260° F.                                                                      280° F.                         ______________________________________                                        I      15%       25%     60%     79%   80%                                           19%       31%     62%     78%   79%                                    II     13%       21%     54%     78%   80%                                           18%       29%     57%     77%   76%                                    III    14%       26%     50%     78%   80%                                           18%       30%     58%     82%   84%                                    A       4%        7%     16%     79%   80%                                            8%       15%     26%     80%   81%                                    B      10%       16%     32%     77%   82%                                           13%       21%     40%     76%   79%                                    ______________________________________                                    

It should be understood that the detailed description and specificexamples which indicate the presently preferred embodiments of theinvention are given by way of illustration only since various changesand modifications within the spirit and scope of the invention willbecome apparent to those of ordinary skill in the art upon review of theabove detailed description and examples.

From the above values and after review of FIGS. II and III it can bereadily seen that the shrink/temperature curves in both the transverseand longitudinal directions of embodiments I, II and III are moregradual and linear than the curves for prior art films A and B. Ofparticular note is the fact that the three embodiments of the presentinvention possess greater free shrink values for a given temperature ascompared to the free shrink values for prior art films A and B.

In view of the above I claim:
 1. An oriented heat sealable multilayerfilm comprising:a cross-linked core layer consisting essentially of alinear low density polyethylene; and two cross-linked surface layerseach comprising a three component blend of (1) a linear low densitypolyethylene, (2) a linear medium density polyethylene and (3) anethylene vinyl acetate copolymer.
 2. An oriented heat sealable threelayer packaging film comprising:a cross-linked core layer consistingessentially of a linear low density polyethylene; two cross-linkedsurface layers each comprising a three component blend of (1) from about40% to about 60%, by weight, of a linear low density polyethylene, (2)from about 20% to about 30%, by weight, of a linear medium densitypolyethylene and (3) from about 20% to about 30%, by weight, of anethylene vinyl acetate copolymer.
 3. An oriented heat sealable threelayer packaging film comprising:a cross-linked core layer consistingessentially of a linear low density polyethylene having a density ofabout 0.920 grams per cubic centimeter; and two cross-linked surfacelayers each consisting essentially of a three component blend of (1)about 50%, by weight, of a linear low density polyethylene having adensity of about 0.920 grams per cubic centimeter, (2) about 25%, byweight, of a linear medium density polyethylene having a density ofabout 0.935 grams per cubic centimeter and (3) about 25%, by weight, ofan ethylene vinyl acetate copolymer comprising from about 3.3% to about4.1% vinyl acetate derived units, said copolymer having a density offrom about 0.9232 to about 0.9250 grams per cubic centimeter.
 4. Thefilm of claims 1 or 2 or 3 having longitudinal and transverseshrink/temperature curves substantially as shown in FIGS. 2 and 3 forembodiments I, II and III.
 5. The film of claims 1 or 2 or 3 havinglongitudinal and transverse shrink values of greater than 10% at 200° F.6. The film of claim 2 wherein said ethylene vinyl acetate copolymercomprises from about 2%, by weight, to about 18%, by weight, of vinylacetate derived units.
 7. The film of claim 2 wherein said vinyl acetatecopolymer comprises from about 2%, by weight, to about 10%, by weight,of vinyl acetate derived units.
 8. The film of claim 2 wherein saidethylene vinyl acetate copolymer comprises from about 2%, by weight, toabout 5%, by weight, of vinyl acetate derived units.
 9. The film ofclaims 2 or 3 having a melt flow at 230° C. and 21,600 grams load offrom about 1.0 to about 10.0 grams per ten minutes.
 10. The film ofclaims 2 or 3 having a melt flow at 230° C. and 21,600 grams load offrom about 2.0 to about 5.0 grams per ten minutes.
 11. The film ofclaims 2 or 3 having a melt flow at 230° C. and 21,600 grams load ofabout 3.0 grams per ten minutes.
 12. The film of claims 2 or 3 having anaverage ball burst value of from about 17 to about 22 centimeters timeskilograms.
 13. The film of claims 2 or 3 having an average ball burstvalue of greater than from about 19 centimeters times kilograms.
 14. Thefilm of claims 2 or 3 which has been cross-linked with from about 1.0 MRto about 6.0 MR of irradiation.
 15. The film of claims 2 or 3 which hasbeen cross-linked with from about 1.5 MR to about 3.5 MR of irradiation.16. The film of claims 2 or 3 which has been cross-linked with about 2.5MR of irradiation.
 17. The film of claims 2 or 3 which has been orientedby racking at a racking ratio of from about 3.0 to about 6.0 in both thelongitudinal and transverse directions.
 18. The film of claims 2 or 3which has been oriented by racking at a ratio of greater than about 4.5times in both the longitudinal directions.
 19. The film of claims 2 or 3which has been oriented by racking at a racking ratio of from about 4.5to about 5.5 in both the longitudinal and transverse directions.
 20. Thefilm of claims 2 or 3 which have been oriented by racking at a rackingratio of about 5.0 in both the longitudinal and transverse directions.